In many industries, it is often desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
Unfortunately, the heat and stress associated with normal wear of many polymeric compositions leads to the breaking of carbon-carbon bonds which, in turn, leads to the generation of free radicals. This is often the case with rubber tread compositions, and leads to increased hysteresis loss and other problems associated with wear and the deterioration of the tread composition.
The free radicals in the rubber polymer tread compositions are known to react with undesirable unsaturated sites, e.g., double bonds, of the polymer or with oxygen. In either case, the reaction of the free radicals are deleterious to the composition. When free radicals generated from the stress inherent in the use of the tread composition react with unsaturated sites, the composition is known to increase its Mooney viscosity, which in turn, decreases wear. The composition itself may even turn brittle.
When the free radicals generated react with oxygen, peroxides are formed in the composition which can generate additional free radicals and continue the reaction with undesired unsaturated sites. Alternatively, the additional free radicals can react with natural rubber and cleave the polymer chain, thereby decreasing the molecular weight of the natural rubber and ultimately destroying the polymer composition.
An important parameter in determining whether a composition will have improved wear resistance and/or abrasion resistance is the filler dispersion of the rubber composition. In the art, filler dispersion of the composition is generally regarded as the uniformity with which the filler is mixed in the polymer matrix. As such, it is generally recognized that the filler dispersion of a filler-reinforced rubber composition is sometimes related to the change in storage modulus or the "Payne effect" of the composition. By decreasing the change in storage modulus, i.e., reducing the "Payne effect", the filler dispersion of the composition may be improved.
Another known parameter used in determining the wear characteristics of a tread composition is the bound rubber content of a filler-reinforced rubber composition. The bound rubber content is generally regarded in the art as the connection or physical interaction between the reinforcing filler, e.g., carbon black, silica, etc., and the elastomer or polymer matrix. In silica-filled rubber compositions, such connections have heretofore been made by adding an alkoxysilane coupling agent, e.g., bis[3-(triethoxysilyl) propyl]tetrasulfide (Si69), which also has sulfur linkage. The sulfur linkages attach themselves to the polymer while the alkoxy functionalities are capable of linking onto the silica fillers. In carbon black-filled elastomer compositions, the exact process by which carbon black generates a high bound rubber content is still not fully understood.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such pneumatic tires.
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as a sulfur-containing vulcanizing (that is, curing) system.
Previous attempts at preparing readily processable, vulcanizable, silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused upon the sequence of adding ingredients during mixing (Bomal, et al., Influence of Mixing procedures on the Properties of a Silica Reinforced Agricultural Tire Tread, May 1992), the addition of de-agglomeration agents such as zinc methacrylate and zinc octoate, or SBR-silica coupling agents such as mercapto propyl trimethoxy silane (Hewitt, Processing Technology of Silica Reinforced SBR, Elastomerics, pp 33-37, March 1981), and the use of bis[3-(triethoxysilyl)propyl]tetrasulfide (Si69) processing aid (Degussa, PPG).
Precipitated silica has also been increasingly used as a reinforcing particulate filler in rubber components of tires and mechanical goods. Silica-filled rubber stocks, however, exhibit relatively low bound rubber content and poor processability.
It is therefore desirable to develop a vulcanizable, filler-loaded elastomeric stock composition useful for tire treads and the like, having improved filler dispersion (i.e., reduced "Payne effect") and improved bound rubber content, which leads to reduced hysteresis and improved processability by reducing Mooney viscosity, while maintaining good physical properties. It is also believed desirable to develop a composition that will trap and, thereby, stabilize free radicals generated upon wear of the tread composition, thus preventing them from reacting with double bonds, oxygen and the like.